Hard magnets (permanent magnets) are used in motors and various other equipment. Above all, automobile motors, etc. have the strongest demands in terms of miniature size and high output. This type of hard magnet with superior magnetic characteristics is of course demanded at a low price due to worldwide competition. First, from the viewpoint of high performance, the development of a RfeB-type magnet (rare-earth magnet), made from a rare-earth element (R), Boron (B) and Iron (Fe), has up until now been popular.
In the way of these RFeB-type magnets, RFeB-type isotropic magnet alloys that possess magnetic isotropy have been released, for example U.S. Pat. No. 4,851,058 (referred to as “Prior Technology 1” below) and U.S. Pat. No. 5,411,608 (referred to as “Prior Technology 2” below).
Specifically, in Prior Technology 1, a magnet alloy with approximately 10-40 at % Nd, Pr or Nd and Pr, approximately 50-90 at % Fe, and approximately 0.5-10 at % B is disclosed. In Prior Technology 2, a magnet alloy is disclosed with the content of 12-40 at % Nd, Pr or Nd and Pr, not more than 10 at % Co, 3-8 at % B, and a remainder of Fe. In Prior Technology 2, through the addition of Co, the heat resistance was increased upon an increase in Curie temperature. Both of these Prior Technologies result in magnet powder with the above-mentioned composition through a type of quenching solidification called melt-spun method.
This magnet powder has many uses industrially as a raw material powder for bonded magnets (hard magnets). The bonded magnets are obtained by, for example, first making pellets from this magnetic powder and a resin binder, then inserting the pellets into a die for molding, and then carrying out compression molding. In general, when the pellets are inserted into the mold, if the degree of fluidity (s·g−1) is small, the time required for pellet insertion will be short, and thus the productivity of the bonded magnets will be increased. Moreover, if the apparent density of the pellets is high, uniform pellet insertion into the mold becomes possible and the failure rate of the bonded magnets can be reduced. Therefore, with a small degree of fluidity and a high apparent density, the cost reduction of the bonded magnet can be achieved resulting in a very economical magnet. The degree of fluidity and apparent density depend on the particle shape of the magnet powder, that is to say that ideal degree of fluidity and apparent density are obtained from a spherical shape.
However, the shape of the magnetic powder particles produced by the above-mentioned Prior Technology 1 and 2 is a ribbon shape with thickness of 20-50 μm, and when compared to the case of a round shape, have a large degree of fluidity and small apparent density. Of course on can think to crush this ribbon shape to make a spherical shape thus making a small degree of fluidity and a large apparent density. However, this would increase the number of production steps and even if a spherical shape is achieved, in fact, it is difficult to effectively make the degree of fluidity small and the apparent density large. This is because the original particle size is at maximum 50 μm and in general it is easy to get adhesion/cohesion with powder not more than 50 μm.
Moreover, the situation is the same even with a SmFeN-type magnet powder, which has a different composition than that mentioned above. Even if the magnet powder particle size is close to a spherical shape, and it is made to be a fine powder with an average particle diameter of 1-5 μm, it is easy to get adhesion/cohesion and the degree of fluidity and apparent density become worse.
Therefore, alternative production methods of magnet powder, other than the above-mentioned melt spinning method, include the HDDR (hydrogenation-disproportionation-desorption-recombination) processing method and the d-HDDR processing method. The magnet powder acquired by these methods have virtually a spherical particle shape and therefore have the degree of fluidity and apparent density that are much superior to the magnet powder acquired by the above-mentioned Prior Technology 1 and 2.
The HDDR processing method is used to produce RFeB-type isotropic magnet powder and RFeB-type anisotropic magnet powder, and it generally has two production steps. That is to say, the first step of 3-phase decomposition (disproportionation) reaction (the hydrogenation step) is carried out while maintained at 773-1273 K in a hydrogen gas environment on the order of 100 kPa (1 atm), and after that is the dehydrogenation step (the second step) where dehydrogenation occurs under vacuum.
On the other hand, d-HDDR is used predominantly in as a production method for RFeB-type anisotropic magnet powder. As reported in detail in commonly-known literature (Mishima, et. al.: Journal of the Japan Applied Magnetics Society, 24 (2000), p. 407), it is defined as the control of the reaction rate between the RFeB-type alloy and hydrogen when going from room temperature to a high temperature. In detail, the four principal production steps are the low-temperature hydrogenation step (step 1) where hydrogen is sufficiently absorbed into the RFeB-type alloy at room temperature, the high-temperature hydrogenation step (step 2) where the 3-phase decomposition (disproportionation) reaction occurs under low hydrogen pressure, the evacuation step (step 3) where hydrogen is decomposed under as high a hydrogen pressure as possible, and the dehydrogenation step (step 4) where the hydrogen is removed from the material. The point which differs from the HDDR process is that through the preparation of multiple production steps with different temperatures and hydrogen pressures, the reaction rate of the RFeB-type alloy and hydrogen can be maintained relatively slow, thus securing homogeneously anisotropic magnet powder.
The following Prior Technologies can be given as examples of these various processes used in magnet powder production methods.
First, the production method of RFeB-type isotropic magnet powder using the HDDR process is disclosed in the Japanese Examined Patent Publication (Kokoku) No. 7-68561 (U.S. Pat. No. 2,041,426: hereafter referred to as Prior Technology 3). According to this Prior Technology 3, the magnet powder is produced by carrying out crushing step and homogenizing heat treatment step, and after that carrying out the above-mentioned HDDR process on an alloy ingot with a main composition of R, Fe and B. Here, in this situation, the two steps prior to the HDDR process (the crushing step and the homogenizing heat treatment step) are essential to obtain an isotropic magnet powder with great magnetic properties. AS is even written in Prior Technology 3, if these two steps are omitted and magnet powder is produced by the direct HDDR processing of RFeB-type cast alloy, the bonded magnet made from the resulting magnet powder will have an extremely low value of coercivity (hereafter referred to as iHc) at a maximum of 0.76 MAm-1.
Additionally, these two steps come with a high cost and uneconomical. In particular, a homogenizing heat treatment step carried out at a high processing temperature between 873-1473 K will double the cost of the HDDR process, thus being very uneconomical.
Next, the RFeB-type anisotropic magnet powder production method, etc. using the HDDR process is disclosed in U.S. Pat. No. 2,576,671 (hereafter referred to as Prior Technology 4), U.S. Pat. No. 2,576,672 (hereafter referred to as Prior Technology 5), U.S. Pat. No. 2,586,198 (hereafter referred to as Prior Technology 6) and U.S. Pat. No.2,586,199 (hereafter referred to as Prior Technology 7).
In an actual example of these Prior Technologies, magnetic powder was produced by carrying out a homogenizing heat treatment step and a crushing step and then carrying out the HDDR treatment on alloy ingot of approximately 10-20 at % R, approximately 5-20 at % B and a remainder of Fe with various additive elements. However, in the examples, the disclosed amount of B in the alloy was disclosed in detail for not more than 10.4 at %, but only disclosed for 10.4 at % and 20 at % for alloys with B content exceeding 10.4 at %. The (BH)max of the anisotropic bonded magnet made from the magnet powder with B composition of 10.4 at % was 83-112 kJ/m3 and the iHc was 0.74-0.97 MA/m, while the (BH)max of the anisotropic bonded magnet made from the magnet powder with B composition of 20 at % was 80-93 kJ/m3 and the iHc was 0.46-0.75 MA/m. These magnetic properties are not nearly sufficient, especially the significant decrease in iHc with a B composition up to 20 at %.
Moreover, in the case of the above-mentioned Prior Technology when the homogenizing heat treatment step is carried out at a high temperature between 1393-1413 K, the cost of production of the magnet powder is high and especially uneconomical. Of course, in this case, the omission of the homogenization heat treatment would reduce the cost, however as in the case of Prior Technology 3, there would be no avoiding a decrease in the iHc properties.
Incidentally, in reality it is very difficult to industrially mass produce isotropic magnet powder using the methods like those in Prior Technologies 4-7. This is because, in order to achieve magnet powder with excellent anisotropy, the temperature during the HDDR process must be controlled very strictly. To be specific, if the HDDR process is not carried out within a ±20 K range of the target temperature then high anisotropic magnet powder cannot be achieved. Additionally, if the amount processed per batch of HDDR process is increased, the generation of heat during the disproportionation reaction between hydrogen and RFeB-type alloy as well as the heat loss during the dehydrogenation will drastically increase, and the atmospheric temperature will become outside of the desired range. As a result of this, the magnetic properties, especially the anisotropy, of the magnet powder massed produced by methods such as Prior Technologies 4-7 are small.
Therefore, when mass producing RFeB-type anisotropic magnet powder, it is recommended that the d-HDDR process is used as disclosed in patents such as Japanese Unexamined Patent Publication No. 2001-148306 (hereafter referred to as Prior Technology 8) or Japanese unexamined Patent Publication No. 2001-76917 (hereafter referred to as Prior Technology 9) etc. With this d-HDDR process, even if the amount processed per batch is increased, a magnet powder having high anisotropy can be obtained. As mentioned above, in the case of d-HDDR processing, because the reaction rate between the RFeB-type alloy and hydrogen is controlled to be slow through a number of processing steps, the amount of heat generation during the disproportionation reaction between the RFeB-type alloy and hydrogen as well as the amount of heat loss during dehydrogenation can be controlled.
However, in the case of Prior Technologies 8 and 9, anisotropic magnet powder was produced by carrying out this d-HDDR processing after performing a homogenizing heat treatment step on an alloy ingot of approximately 12-15 at % R, approximately 6-9 at % B and a remainder of Fe. Because of this, the production cost was high, similar to the above-mentioned production methods.
Up to now the main introduction has been about the cost reduction of hard magnets with high properties, but when considering actual use, it is also important to have excellent properties over time such as corrosion resistance etc. In particular, there is a demand for superior heat resistance, etc. for hard magnets used in a high-temperature environment, such as in motors of household appliances and automobile motors, etc., from the point of view of securing motor reliability, etc.
However, for the above-mentioned rare-earth magnets, it is very easy for the properties of the Fe and R that are the main components of the composition to be reduced by oxidation corrosion etc., and therefore it is difficult to steadily ensure high magnetic properties. Especially in the case that rare-earth magnets are used above room temperature, there is a tendency for the magnetic characteristics to drop off drastically. The permanent demagnetization ratio (%) is usually used to index a magnet's change over time, and in the case of the prior rare-earth magnets, most of them had permanent demagnetization ratios exceeding 10%. Moreover, when held below a certain temperature for an extended period of time (over 1000 hours), the permanent demagnetization ratio does not return to its original level when remagnetized and the percentage of magnetic flux is reduced.